1. Field of the Invention
This invention relates generally to a fuel cell system that employs an algorithm for determining whether product water in a fuel cell stack is likely to freeze after system shut-down, and if so, perform a stack purge and, more particularly, to a fuel cell system that employs an algorithm for periodically determining whether sub-zero conditions may exist after system shut-down so as to selectively perform a stack purge.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. The bipolar plates are made of a conductive material, such as stainless steel, so that they conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
As is well understood in the art, fuel cells operate with a certain relative humidity based on the operating temperature of the fuel cell stack and the pressure within the stack to provide efficient stack operation. Also, the stack produces product water. Therefore, when the fuel cell stack is shut down, the MEAs within the stack have a certain amount of moisture. If the fuel cell system happens to be in a sub-zero environment, this moisture can freeze, which may damage the MEAs, diffusion media, plates and/or gaskets. Therefore, it is known in the art to dry the fuel cell stack and membranes therein at system shut-down to prevent the stack from being damaged as a result of freezing.
In one known technique, dry air is forced through the stack by the compressor to provide water purging and stack drying. Most of the moisture is present in the cathode side of the MEAs because of the product water, however, there is some moisture in the anode side of the MEAs because the MEA is wet and moisture diffuses through the MEA from the cathode side to the anode side. Therefore, stack drying techniques also direct the cathode air through the anode channels for drying purposes. Various methodologies are known in the art for providing the anode flow channel and/or cathode flow channel purge. In one known technique, the fuel cell stack power is used to operate the compressor to provide the air purge. However, this requires fuel to do this. Alternately, battery energy can be used to provide the purge. However, this reduces the battery's stored energy.
It is preferred that the reactant gas flow channel purge not be performed if the product water in the stack is not going to freeze because purging dries out the membranes, potentially reducing their life, and requires energy to operate the compressor to provide the purge. Therefore, it would be desirable to be able to predict whether the vehicle will be subject to freezing conditions after it is shut down.